This application is based on Japanese Patent Application No. 2003-022132 filed on Jan. 30, 2003, the contents of which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a taking lens system, and more particularly to a compact, high-performance taking lens system suitable for use in a digital input apparatus (such as a digital still camera or digital video camera) that takes in an image of a subject by means of a solid-state image sensor.
2. Description of the Prior Art
In recent years, as personal computers and the like become more and more popular, digital still cameras and digital video cameras (hereinafter collectively referred to as xe2x80x9cdigital camerasxe2x80x9d), which permit easy entry of image data into digital equipment, have been becoming more and more popular for personal use among general users. Such digital cameras are expected to become even more widespread in the future.
Solid-state image sensors, such as CCDs (charge-coupled devices), used in digital cameras have been made increasingly small, and accordingly further miniaturization is sought in digital cameras themselves. As a result, miniaturization is eagerly sought also in taking lens systems, which occupy the largest volumes in digital input devices. The easiest way to make a taking lens system smaller is to make a solid-state image sensor smaller. However, this involves making photosensitive elements smaller, and thus makes the solid-state image sensor more difficult to fabricate, and in addition requires higher performance in the taking lens system.
On the other hand, making a taking lens system smaller while keeping the size of a solid-state image sensor unchanged inevitably brings the exit pupil position closer to the image plane. When the exit pupil position is brought closer to the image plane, the off-axial rays exiting from the taking lens system are obliquely incident on the image plane. This makes it impossible to make the most of the light-condensing ability of the microlenses provided in front of the solid-state image sensor, with the result that the obtained image shows extremely uneven brightness between in a central portion and in a peripheral portion thereof. This can be avoided by bringing the exit pupil position of the taking lens system farther away from the image plane, but this inevitably makes the taking lens system as a whole unduly large.
Furthermore, in keen competition for lower prices in recent years, lower costs have been increasingly eagerly sought in taking lens systems. Moreover, to cope with the trend toward denser and denser solid-state image sensors in recent years, increasingly high performance has been sought in taking lens systems. To meet these requirements, Japanese Patent Applications Laid-Open Nos. 2000-171697 and 2002-221657 propose taking lens systems composed of four lens elements for use with a solid-state image sensor.
However, the taking lens system proposed in Japanese Patent Application Laid-Open No. 2000-171697 mentioned above has an angle of view of about 28 degrees, which is insufficient for it to be used as a taking lens system. On the other hand, the taking lens system proposed in Japanese Patent Application Laid-Open No. 2002-221657 mentioned above has a total length roughly equal to three times its focal length, and thus is not satisfactorily compact. Moreover, this taking lens system uses as many as two glass lens elements, which unduly increase its costs.
An object of the present invention is to provide a taking lens system for use with a solid-state image sensor which is low-cost and compact but which nevertheless offers satisfactory optical performance.
To achieve the above object, according to one aspect of the present invention, a taking lens system composed of four lens elements for forming an image on a solid-state image sensor is provided with, from the object side, a first lens element having a positive optical power, an aperture stop, a second lens element having a positive optical power, a third lens element having a negative optical power, and a fourth lens element having a negative optical power. Here, the following condition is fulfilled:
0.5 less than f/f1 less than 1.6
where
f represents the focal length of the entire taking lens system; and
f1 represents the focal length of the first lens element.
According to another aspect of the present invention, a taking lens system composed of four lens elements for forming an image on a solid-state image sensor is provided with, from the object side, a front lens unit having a positive optical power, an aperture stop, and a rear lens unit. Here, the front lens unit is composed solely of a first lens element having a positive optical power, and the rear lens unit is composed of, from the object side, a second lens element having a positive optical power, a third lens element having a negative optical power, and a fourth lens element having a negative optical power. Moreover, the following condition is fulfilled:
xe2x88x920.9 less than f/fR less than 0.4
where
f represents the focal length of the entire taking lens system; and
fR represents the focal length of the rear lens unit.
According to still another aspect of the present invention, a taking lens system composed of four lens elements for forming an image on a solid-state image sensor is provided with, from an object side, a first lens element having a positive optical power, an aperture stop, a second lens element having a positive optical power, a third lens element having a negative optical power, and a fourth lens element having a negative optical power. Here, the first lens element is a positive lens element convex to the object side, the second lens element is a positive meniscus lens element convex to an image side, the third lens element is a negative meniscus lens element convex to the image side, and the fourth lens element is a negative lens element concave to the image side and having an aspherical surface at least on the image side thereof